The organization of cellular and extracellular components appeared similar and was distinctive in 3 endocardial papillary elastofibromas studied. Each tumor papilla contained: 1) a dense, central core of collagen and elastic tissue; 2) a peripheral, myxomatous layer with deposits of acid mucopolysaccharides, and 3) an overlying, hyperplastic layer of endothelial cells. The cells in all 3 zones had numerous cytoplasmic filaments, 100 A in diameter, and dilated cisterns of endoplasmic reticulum; endothelial cells also had intercellular junctions and numerous pinocytotic vesicles. The myxomatous stroma varied from amorphous to fibrillar, and the collagenous cores showed focal degeneration.